On the chemical markers of pyroxenite contributions in continental basalts in Eastern China: Implications for source lithology and the origin of basalts

Although it is widely recognized that mantle peridotite is compositionally heterogeneous, primitive mantle-like peridotite is commonly considered the potential peridotite source when identifying the source lithology of oceanic and continental basalts. Here, a simple equation that compatible with thermodynamic result (Ol (wt%) = 144.27ln((Fe + Mg + Ca + 2Na) / Si), mol ratio) is introduced to distinguish peridotitic and pyroxenitic rocks. Meanwhile, the basic assumptions, alternative interpretations and uncertainties of many of the literature markers for pyroxenite contributions in basalts are briefly summarized, and some detailed comments on these markers are presented using literature data on olivines and basalts from Eastern China. Both high-CaO (> 0.1 wt%) and low-CaO (< 0.1 wt%) olivines in these continental basalts are likely generated by a xenocryst diffusive re-equilibration process, suggesting that the olivine chemistry method may not be suitable for the source lithology identification of the basalts. For the Cenozoic and Mesozoic continental basalts in Eastern China, nearly all of the markers for pyroxenite sources based on whole-rock major and/or trace elements might be compromised by refertilized/metasomatized peridotite sources. These uncertainties also exist for many other oceanic and continental basalts. However, the FC3MS (FeOT/CaO-3 ∗ MgO/SiO2, all in wt%) value, combined with those of Na2O + K2O, Mg#, MgO and La/Yb, suggests that Cenozoic OIB-type basalts in Eastern China are mostly primary pyroxenite-derived products, and caution should be paid when interpreting the petrological and geological implications of these basalts, particularly that of the low-MgO pyroxenite-derived basalts. At the normal mantle potential temperature (~ 1350–1400 °C), the pyroxenite source can produce melts that explain all of the compositional characteristics of the Cenozoic basalts, suggesting that the origin temperature and pressure of the Cenozoic basalts determined by the traditional peridotite model are overestimated. For the Mesozoic basalts, compositionally diverse peridotite sources can account for their petrogenesis, but the pyroxenite source cannot be excluded. We acknowledge that the pyroxenite source may represent crustal materials (continental or oceanic crust) recycling and/or mantle metasomatism, which have important geodynamic implications, but further work is needed to determine the origin of the pyroxenite.